The six studies presented in this thesis investigated the extent to which people compare to others when making a range of health-related judgements and decisions and aimed to identify the cognitive mechanisms used in this comparison process. A key question was whether biases found in specific judgements and decisions, such as deciding to seek help when it is not needed or not seeking help when it is needed, could be explained by social comparison effects. It was found that participants compared to others using rank-based strategies when making judgements and decisions about mental and physical health symptoms and when judging their health in general (Study 1, 2 and 4). Social comparison effects were generally small to medium in size (average Cohen's f2 = 0.09, range = 0.01-0.39). Health-related help-seeking accuracy was associated with how participants believed their experience of symptoms compared to that of others. Participants were four times more likely to seek help when it was not needed if they believed that they experienced symptoms more frequently than others, and two to three times more likely not to seek help when it was needed if they believed that they experienced symptoms less frequently than others (Study 1). However, participants' beliefs about how their sleep compared to that of others had little influence on their sleep-related judgement and decision-making (Study 3). There was no evidence that participants' beliefs about how much support they received relative to others was associated with perceptions of this support (Study 5 and 6). The findings have implications for the development of both interventions that may improve accuracy in health help-seeking decisions and social norms-based interventions, the measurement of comparison effects, self-rated health and social support, social comparison theory, and models of symptom appraisal and health-related help-seeking.